Duct assessment and leak sealing method for ducts transporting gas

ABSTRACT

The present invention relates to a method of assessing the condition of a duct along which a gas is flowing. The method comprises producing one or more sensor carrying elements, wherein the elements are formed to have selected parameters; introducing the sensor carrying elements into the flowing gas; and selecting the parameters of the sensor carrying elements such that the elements are transported along the duct by saltation. The present invention also relates to a method of reducing leakage through a leak in the duct. In this respect, the method comprises producing sealing elements, wherein the sealing elements are formed to have selected parameters; introducing a plurality of the sealing elements into the flowing gas; and selecting the parameters of the sealing elements such that the elements are transported along the duct by saltation and such that, at the locality of the leak, at least one of said sealing elements is captured by a pressure differential associated with the leak and is thereby drawn to and held in position at the leak for stemming or sealing it.

The present invention relates to a method of assessing the condition ofa duct along which a gas is flowing and a method of reducing leakagethrough a leak in a duct along which a gas is flowing.

Ducts or pipes are used extensively in industry to carry liquids andgases, often under high pressure. Once a duct or pipe has beeninstalled, small fractures and/or holes can appear over time in the ductor pipe wall or at the joints between connecting sections of ducts.These are the result of various factors including corrosion, poorinstallation, manufacturing defects or mechanical damage; thus, leakageoccurs.

However, it is usually the case, for one reason or another, thatsignificant areas of the liquid or gas carrying ducts, once installed aspart of a pipework system, become substantially inaccessible. Forexample, mains water distribution systems employ vast lengths of buriedpipework which involves expensive and time consuming excavation toexpose areas of pipe from which leakage is suspected.

The present Applicants have developed a number of techniques forlocating, sealing and stemming leaks from liquid carrying ducts,examples of the techniques are disclosed in WO-A-01/086191 andWO-A-03/093713. The techniques uses sealing elements having a buoyancysimilar to the liquid in the duct, i.e a neutral buoyancy. A pluralityof the sealing elements are introduced into the duct and float alongwith the liquid. Due to the turbulence in the liquid, the sealingelements always encounter a leak no matter its location around the crosssection of the duct and can therefore be captured by a pressuredifferential associated with the leak. Depending on the particulartechnique, the element can assist in locating the leak or can assist instemming or sealing it. In an alternative, the elements can carrysensors so that the condition of the duct can be assessed by remotemonitoring or recording.

The above described techniques are very effective in the case of a ductcarrying liquid.

However, when these techniques are applied to the case of a ductcarrying gas, it has been found that close matching of the buoyancy ofthe element to the gas density is very difficult. Moreover, even if suchmatching can be achieved, it has been found that elements of such lowdensity do not have sufficient strength to maintain their form at theleak for location or sealing purposes. In addition, it has been foundthat such elements do not encounter leaks around the entire crosssection of the duct and that the movement of the elements tend to bedominated by the drag forces exerted by the gas such that the elementare not necessarily drawn to the leak at all, particular for leakslocated towards and upper part of the duct.

It is an object of the present invention to provide a technique ofassessing the condition of gas transporting ducts and of locating,sealing or stemming leaks from gas transporting ducts.

According to a first aspect of the present invention there is provided amethod of assessing the condition of a duct along which a gas isflowing, the method comprising:

producing one or more sensor carrying elements, wherein the elements areformed to have selected parameters;

introducing one or more of said sensor carrying elements into the gasflowing along the duct;

and selecting said parameters of the sensor carrying elements such thatthe elements are transported along the duct by saltation.

According to a second aspect of the present invention there is provideda method of reducing leakage through a leak in a duct along which a gasis flowing, the method comprising:

producing sealing elements, wherein the sealing elements are formed tohave selected parameters;

introducing a plurality of said sealing elements into the gas flowingalong the duct;

selecting said parameters of the sensor carrying elements such that theelements are transported along the duct by saltation and such that, atthe locality of the leak, at least one of said sealing elements iscaptured by a pressure differential associated with the leak and isthereby drawn to and held in position at the leak for stemming orsealing it.

By selecting the parameters of the elements such that the elements aretransported along the duct, it is possible to ensure that in a gastransporting duct, a complete sweep of the entire volume of the gasfilled duct can be made, and hence a complete sweep of the duct wall canbe made. Accordingly, full analysis of the duct and/or sealing of leaksin the entire duct wall may be greatly facilitated for any orientationof the duct. Thus, having the elements transported along the duct bysaltation, there is a bouncing ricocheting motion in which the transportof the elements is dominated by the frequent collisions with the wall ofthe duct.

By selecting the parameters of the elements appropriately, it ispossible to optimise the transport mechanism of saltation to make theaforementioned complete sweep, and it is possible to optimise the energytransfer from the gas flow to the elements so that the number ofelements required to have a sweep of the entire duct volume isminimized.

The present invention will now be described by way of example only. Thepresent invention intends to provide a technique of assessing thecondition of a duct along which a gas is flowing for transport from onelocation to another and of locating, sealing or stemming leaks from sucha duct.

As noted above, known elements introduced into a gas carrying duct,which had a density similar to the density of the gas, did not havesufficient strength to maintain their form at the leak for location orsealing purposes. Whilst stronger and hence heavier elements overcamethis problem, another problem resulted in that such heavier elementscould not be transported along the duct with the gas flow and could notbe transported along the duct in a manner to ensure a complete sweep ofthe entire cross section of the duct, that is to say, all portions ofthe duct wall, top bottom and the sides (for a circular cross sectionduct). Hereinafter, the bottom of the duct will be referred to as the 6o'clock position and the top of the duct will be referred to as the 12o'clock position.

It has been found that by forming the elements with particular selectedparameters, it is possible to provide elements which are transportedalong the duct with the gas flow by saltation. Full saltation isconsidered to be the situation where the element rebounds from the ductwall at the 12 o'clock position as well as elsewhere. Of course thenumber of rebounds of an element near the 12 o'clock position will besignificantly less that the number of rebounds near the 6 o'clockposition. Effective saltation is considered to be when elements reboundnear the 6 o'clock position substantially not more than 10 times as theyrebound near the 12 o'clock position.

Saltation is a type of particle transport in which loose material in afluid flow is removed from a surface, carried along by the fluid flow,before being returned to the surface, from which it may rebound orricochet so the motion is repeated. Amongst examples of saltation arepebble transport in rivers, and sand grains blowing over dunes.

Looking at pebble transport, at low flow velocities, the pebble rollsdownstream whilst at high flow velocities, the lift and moment exertedby the fluid flow on the material is enough to pull the pebble away fromthe river bed and into the river flow. As the pebble moves into thefaster flow of the river away from the bed, the velocity differencebetween the pebble and the river flow decreases and so lift decreases.When the pebble weight is greater than the lift force, the pebble sinksback towards the surface. The pebble continues its transport path havingfrequent collisions with the bed thereby producing a discontinuousricocheting progression along the river. Thus, a parabolic trajectoryflight in the fluid between rebounds characterises transport bysaltation with little deviation due to turbulence.

When being transported by saltation, the elements are driven along theduct in the flow direction, i.e. the longitudinal or downstreamdirection of the gas flow. The drag forces acting on the elements by thegas result in the elements gaining kinetic energy in the longitudinaldirection. Part of this velocity is converted to a velocity in thetransverse or crosstream direction of the gas flow when the elementscollide or impact with the duct wall.

For effective assessment of the condition of the duct and/or forlocation or reduction of leakage from the duct, it is important that theelements can encounter at least a large proportion of the entire crosssection of the duct carrying gas, that is to say, the elements should bemaking a complete sweep of the entire volume of the duct such that everypart of the duct volume has a sufficiency of elements passing throughit. In terms of sealing, there needs to be a sufficient intensity (permeter squared) of all collisions to ensure that a leak, should it exist,at any position of the duct will be sealed. Thus, if the transport ofthe elements by saltation is to be effective, the elements need to beable to rebound from the 6 o'clock position of the duct wall sufficientstrongly to reach towards the 12 o'clock position of the duct wall andto rebound in a reasonably random manner. The critical volume to reachis towards the 12 o'clock position where the effects of gravity are mostdetrimental to making a complete sweep.

It has been found with the present invention that for effectivesaltation a number of distinct parameters affect whether a substantiallycomplete sweep can be made. In particular, the elements need to haveparticular selected parameters.

As a crude model, one limit that can be considered for saltation is alower limit of the element velocity after rebound using conservation ofenergy. In the case where the velocity of the element is purely in thetransverse (or vertical) direction and all of the kinetic energy isconverted into potential energy then the maximum height that the elementwill reach is governed by:½mv ² =mgh  1

where:

m=mass of the element

v=transverse velocity of the element at rebound

g=acceleration due to gravity;

h=maximum height reached by the element.

If h set to be equal to the diameter d of the duct, then substituting dinto equation 1 leads to:V _(min)=4.43√d  2

where:

-   -   V_(min)=minimum vertical velocity to achieve a complete sweep        using saltation.

Therefore, if a complete sweep using elements being transported bysaltation is to be achieved in a larger duct, a greater value of V_(min)is required.

However, the above equations assume that the element has all of itskinetic energy directed vertically. In practice this is unlikely to bethe case. The element gains its kinetic energy from the flow of gasalong the duct. Whilst there is some turbulent flow in the duct, theflow is dominated by the longitudinal flow in the direction of the duct.Thus, the driving force acting on the element is also dominant in thisdirection. The event which converts this longitudinal motion intotransverse motion is the collision between the element and the ductwall.

To convert longitudinal velocity to transverse velocity, the elementneeds to be liable to random direction changes during the rebound andmust also conserve energy such that longitudinal velocity is convertedto transverse velocity.

One way to achieve this is to make sure the elements bounce of the wallssuch they there is not too great a disparity in the velocity before andafter impact with the wall. By selecting the material of the elementscarefully to have a high coefficient of restitution, the disparity canbe minimised thereby aiding saltation. The coefficient of restitutioncan be expressed for an oblique collision ase=V _(f) sin θ_(f) /V _(i) sin θ_(i)  2

-   where: V_(f) sin θ_(f) is the component of the rebound velocity    normal to the collision    -   V_(i) sin θ_(i) is the component of the incident velocity normal        to the collision.

Materials with high coefficients of restitution such as butadiene rubberare effective in maintaining the velocity of the element duringsaltation. Butadiene also has a high coefficient of friction, whichencourages spin thus increasing the random direction of the rebound.Angular elements such as cubes are also preferred as they rebound in anirregular fashion.

It has been found that the rebound events from cubes of natural rubbercross-linked with butadiene rubber routinely result in >90%rebound/incident velocity, with the deviations of these bounce eventsbeing up to 50 degrees better than a perfect reflection, the averagedeviation from the perfect reflection being 14 degrees.

If it is assumed that the maximum velocity of the element is equal tothe gas velocity prior to impact and that the rebound is 90% of thatvelocity at 50 degrees to the horizontal, then the minimum gas velocityto achieve saltation is expressed by:v _(gas) =V _(min)/0.9×sin 50°  4

Thus, the minimum gas velocity to achieve saltation is 6.43√d, althoughit should be recognized that this figure is approximate as it is theresult of an empirical study on one particular type of element.Therefore, a minimum gas velocity greater than 6√d is consideredsufficient.

In the example of a 150 mm diameter duct, the minimum gas velocity toachieve some degree of saltation is 2.5 meters per second. Goodsaltation, whereby the average element will reach the 12 o'clockposition can be achieved at 8 meters per second.

There is no theoretical limit to the duct diameter to achieve saltation.However for larger duct diameters, the pressure and hence gas densitywill dominate the flight of the elements.

The above does not take drag into effect. Drag will act to deceleratethe elements in the transverse direction. The drag acting on the elementis proportional to the cross sectional area of the element (i.e. for aregular shaped element the drag is proportional to the square of theside length) whereas the weight is proportional to the volume of theelement (i.e. for a regular shaped element the weight is proportional tothe cube of the side length). Consequently, as size increases, drag isreduced and the effectiveness of the saltation will improve.

Nevertheless, a consequence of drag is that the value V_(min) must alsoincrease to maintain saltation, although as indicated above, as theweight of the element increases, the deceleration due to drag isreduced. Therefore, the higher the weight of the element relative to thefluid, the better the saltation.

The parameters that affect the effectiveness of saltation can besummarised as follows.

1) Gas Velocity: There is a lower limit of approximately 6√d for the gasflow velocity for the elements to be transported effectively bysaltation. As the speed increases, dispersion of the elements improvesto an optimum level. However, beyond this optimum speed, fluid forcesbegin to dominate the transport and behaviour of the elements.

In this connection, gas velocity and gas pressure are linked. It wasfound that as the gas velocity increases, the number of elementsrequired to achieve entrainment at a leak also increases. This is due tothe kinetic energy required to be removed from the elements to stoptheir progress along the duct and achieve entrainment. Varying the gaspressure has the opposite effect on the number of elements required forentrainment which drops as the pressure increases. This is due to thepressure field in the duct being greater for higher pressures; hence theentrainment force is greater and the number of elements required forentrainment drops.

2) Relative Gas to Element Density: The gas density must be high enoughto provide energy transfer to the element in the form of drag, but notso much that the element becomes suspended in the gas and can not dropout to enable saltation. It has been found that gas densities of greaterthan around 0.05% of the element density and up to around 50% of theelement density are sufficient to drive the elements along. If a highgas density is used, the flow of the element is dominated by suspensionso that as the elements travel along the duct, there are fewer vigorouswall collisions and the move in the lower portion of the pipe crosssection, nearer the 6 o'clock position.

It should also be noted that as the gas pressure increases, the bounceheight reduces due to the high pressure/high density gas slowing thefall of the element. Increasing the element size decreases the distancebetween bounces and increases the bounce height. As drag forcesincrease, the elements gain a greater longitudinal acceleration from thegas, but the transverse deceleration is also increased leading to areduced bounce. This results in a flatter flight trajectory.

3) Element Size: The lower limit depends upon the density of the elementand its ability to drop out of suspension. The upper limit depends uponthe diameter of the duct and is of the order of d/4.

4) Pipeline Diameter: There is no theoretical limit to the ductdiameter. In practice, the pressure and hence gas density will dominatethe flight of the elements to a greater extent for larger ductdiameters. There is an inherent link between the duct diameter and thegas velocity with respect to effective saltation.

5) Wall Roughness: Whilst saltation improves with increased wallroughness, the improvement is minimal unless the roughness is extremesuch as pitting defects. It has been noted that conventional wallroughness ceases to influence saltation for particles greater than 700micron in size.

6) Coefficient of Restitution: The nature of the rebound of the elementis controlled by a number of factors:

the incident velocity

the incident angle

the orientation of an angular element with respect to

the duct wall

the duct wall roughness

the rotational velocity of the element

the coefficient of restitution of the element

the coefficient of friction of the element.

The coefficient of restitution represents the ability of the element torebound from a surface and elements formed from materials that exhibit ahigh coefficient of restitution will conserve more energy after acollision than a material with a low coefficient of restitution.Suitable materials include elastic materials such as rubber.

The coefficient of restitution can be evaluated by dropping the elementvertically onto a surface from a known dropping height h and measuringthe rebound height H of the rebound, the coefficient is represented byE=√(h/H)

Thus, an element with a perfect rebound would have a coefficient ofrestitution of 1.0.

It has been found that the coefficient of restitution is one of theparameters affecting whether effective saltation is obtained andgenerally, the coefficient of restitution should have a value in therange 0.5 to 1.0 and preferably in the range 0.75 to 1.0.

Elements formed with angular geometries, such as cubes, also improve thedegree of direction change from a bounce after an element impacts withthe duct wall. Elements formed with surfaces which provide a highcoefficient of friction with the duct wall also improve spin and hencethe degree of direction change from a bounce after an element impactswith the duct wall.

In the following examples, a number of elements were introduced into aduct having a leak. The size of the element was matched to the size ofthe leak to be sealed.

EXAMPLE 1

Elements were formed as a 5 mm cube of polybutadiene rubber vulcanizedwith sulphur. This material exhibits a high coefficient of restitutionof 0.90. The elements were introduced into a duct of diameter 200 mmwith a gas velocity of 8 m/s at a pressure of 1 bar and were found totravel down the duct in accordance with the saltation transportmechanism and were found to be drawn to the site of a leak in order thatthe leak could be sealed. Further experiments found that such elementscan have a range of between 2 to 10 mm edge length and still beeffective.

EXAMPLE 2

Elements were formed as a 2 mm cube of natural rubber. This materialexhibits a relatively lower coefficient of restitution of 0.65. Theelements were introduced into the a duct of diameter 22 mm with a gasvelocity of 3 m/s at a pressure of 8 bar and were found to travel downthe duct in accordance with the saltation transport mechanism and werefound to be drawn to the site of a leak in order that the leak could besealed.

EXAMPLE 3

The polybutadiene rubber vulcanized with sulphur used in Example 1 wasfound to have a tendency to be weak and could crumble when subjected topressure. Whilst the natural rubber used in Example 2 was much strongerin comparison with polybutadiene rubber, it had a lower coefficient ofrestitution so elements made from this material were not so effectivelytransported by saltation. To overcome these limitations, elements madefrom polybutadiene rubber cross linked with natural rubber were formedas 1, 2, 3, 4, 6 and 10 mm cubes exhibiting a coefficient of restitutionof 0.85. The elements were introduced into a duct of diameter 150 mmwith a gas velocity of 14 m/s at a pressure of 60 bar. The resultsshowed that such elements saltate effectively and were found to be drawnto and could seal leaks effectively.

Whilst it is necessary to produce elements which saltate effectively,other factors that affect the selection of an element used for sealinginclude the softness or compliance of the sealing element to form a goodseal and the stiffness of the sealing element to maintain a seal at highpressures. An element, whether for sealing or otherwise, should beselected to be compatible with the gas being carried in the duct.

Although in the examples described herein, the sealing elements areformed as cubes, a variety of other geometries may be used includingspheres, pyramids, octahedrons and tetrahedrons, (more aerodynamicallyshaped elements with higher drag coefficients such as thistle seedshaped elements, sheets or membranes are more suited to suspension andnot saltation).

Whilst the above has been described primarily in relation to elementssuitable for sealing or stemming leaks in a duct, it is apparent thatthe elements can carry sensors so that one or more elements flowing downa gas carrying duct can provide either stored or on line informationabout the condition of the duct walls.

In general it has been found that a minimum requirement to obtainsaltation is if the various parameters are selected such that for eachrebound at or near the 12 o'clock position of the duct, there are notmore than 20 rebounds at or near a 6 o'clock position of the duct.However, good saltation can be achieved if the various parameters areselected such that for each rebound at or near the 12 o'clock positionof the duct, there are not more than 10 rebounds at or near a 6 o'clockposition of the duct.

It will be appreciated that the present invention has been described byway of example and is capable of considerable modification, the detailsof which will be readily apparent to a person skilled in the art.

The invention claimed is:
 1. A method of assessing the condition of aduct along which a gas is flowing, the method comprising: producing oneor more sensor carrying elements, wherein the elements are formed ofelastomeric material having selected parameters; introducing one or moreof said sensor carrying elements into the gas flowing along the duct;and utilizing said parameters of said sensor carrying elements tofacilitate said elements transport along the duct by saltation.
 2. Amethod according to claim 1 wherein the parameters are selected suchthat during saltation the elements rebound at or near a 6 o'clockposition of the duct not more than 20 times for each rebound at or nearthe 12 o'clock position of the duct.
 3. A method according to claim 1wherein the parameters are selected such that during saltation theelements rebound at or near a 6 o'clock position of the duct not morethan 10 times for each rebound at or near the 12 o'clock position of theduct.
 4. A method according to claim 1 further including the step ofarranging the velocity of the gas flowing in the duct to be greater than6d, where d is the diameter of the duct.
 5. A method according to claim1 wherein one of said parameters is the buoyancy of the elements and theelements are selected to be heavier than neutrally buoyant.
 6. A methodaccording to claim 3 wherein one of said parameters is the density ofsaid elements, and wherein the density of the elements is selected to bein the range of 2 to 2000 times the density of the gas.
 7. A methodaccording to claim 1 wherein one of said parameters is the coefficientof restitution of the elements, and wherein the coefficient ofrestitution is selected to be in the range of 0.5 to 1.0.
 8. A methodaccording to claim 4 wherein the coefficient of restitution is selectedto be in the range of 0.75 to 1.0.
 9. A method according to claim 1wherein the elements are formed of rubber.
 10. A method according toclaim 1 wherein the elements are formed of a polybutadiene rubbercross-linked with natural rubber.
 11. A method according to claim 1wherein one of said parameters is the shape of the elements, and whereinthe shape of the elements is selected to be angular.
 12. The method ofclaim 1, wherein the parameters are selected such that during saltationthe elements rebound at or near a 12 o'clock position.
 13. The method ofclaim 12, wherein the parameters are selected such that during saltationthe elements rebound at or near a 6 o'clock position more than eachrebound at or near the 12 o'clock position of the duct.
 14. The methodof claim 12, wherein the elements are formed of elastomeric material.15. The method of claim 1, wherein the elements are selected to have ahigh coefficient of restitution.
 16. The method of claim 15, wherein theelements further comprise one or more sensors.